1. Field of the Invention
The invention relates to an electronic, preferably proximity-type, switching device, with a first switching device terminal, with a second switching device terminal, with a sensor and an evaluation circuit which contains a proximity indicator which can be externally influenced, an operating voltage supply circuit for making available the required internal operating voltage, and with an electronic switch which can be controlled by the sensor and evaluation circuit, for example, a switching transistor, the sensor and evaluation circuit, on the one hand, and the electronic switch, on the other, being connected essentially in parallel and to the first switching device terminal and the second switching device terminal.
2. Description of Related Art
The switching device underlying the invention has a first switching device terminal and a second switching device terminal; therefore, it is a so-called 2-wire switching device. In one such 2-wire switching device, the power supply voltage (= external operating voltage) must be connected to one switching device terminal, while a load can be connected to the other switching device terminal; then, the power supply voltage can be connected to the load. In the switched-through state, then, the load current flows from one pole of the power supply voltage via the switching device, primarily via the conductively controlled electronic switch, and via the load to the other pole of the power supply voltage (or vice versa).
The sensor and evaluation circuit implemented in the switching device of the type under consideration can be made entirely differently; it is almost irrelevant to the teaching of the invention how it is done. This also applies to the execution of the proximity indicator and to the execution of the operating voltage supply circuit. Conventionally, part of the proximity indicator, at least part of the operating voltage supply circuit and at least part of what otherwise belongs to the sensor and evaluation circuit are implemented in an IC (integrated circuit). But this need not necessarily be the case. In the electronic switching device under consideration, even if it is no longer conventional at present, the sensor and evaluation circuit can be built completely from discrete components.
Finally, with reference to the electronic switching device underlying the invention, it was stated at the beginning that the sensor and evaluation circuit, on the one hand, and the electronic switch, on the other, are connected at least essentially in parallel and to the first switching device terminal and the second switching device terminal. "Connected at least essentially in parallel" means that the switching device, with reference to the switching device terminals, has essentially two current paths, specifically, one current path via the sensor and evaluation circuit, and the other current path via the electronic switch. Consequently, the description "connected at least essentially in parallel" also includes an embodiment in which, for example, in series to the electronic switch, there is a Zener diode or polarity reversal protection diode or a short circuit detection resistor, this series connection being parallel to the sensor and evaluation circuit.
Electronic switching devices of the above described type are made without contacts, and for more than twenty years have been increasingly used instead of electrical, mechanically operated switching devices, which are made with contacts, especially in electrical and electronic measurement and control circuits. This applies especially to proximity switches, i.e., for electronic switching devices which work by proximity. With these switching devices, it is indicated whether an influencing element to which the corresponding proximity switch is sensitive has approached the proximity switch closely enough. Specifically, if an influencing element to which the proximity switch is sensitive has approached the proximity switch near enough, the proximity indicator of the electronic switch reverses; in a switching device which is made as a make contact, the electronic switch which was first nonconductive becomes conductive, while in a switching device made as a break contact, the electronic switch which was initially conductive now blocks. (With switching devices of the type under consideration, it can also be indicated whether a physical quantity of an influencing medium to which the switching device is sensitive has reached a corresponding value.)
In electronic switching devices which can be connected via an outer lead to one pole of a power supply voltage, and only via another outer lead to one terminal of a load, i.e., in 2-wire switching devices, it is not easy to make available the internally required operating voltage (= internal operating voltage) or the required operating current because the internal operating voltage or operating current must be made available both in the conductive state and also in the blocked state of the switching device.
It is trivial whether making available an internal operating voltage or operating current is involved, because internally, especially for the proximity indicator and other circuit parts of the sensor and evaluation circuit, electric power is of course needed; therefore, both an internal operating voltage and also an operating current are required (compare the detailed description of this matter in published German Patent Application No. 23 30 233, especially column 5, line 68, to column 6, line 33).
In terms of their function as switching devices, in 2-wire switching devices, in the conductive state, almost no voltage drop will occur, and in the blocked state, almost no current will flow. But, since if no voltage drop were to occur in the 2-wire switching devices in the conductive state, no internal operating voltage could be obtained, and if in the blocked state no current were to flow, no operating current could be obtained, it applies to all electronic switching devices with only two outer wires that, in the conductive state a voltage drop occurs, and in the blocked state a residual current flows.
It follows then from what has been explained above that, if a voltage drop occurs in the conductive state in electronic 2-wire switching devices, unwantedly, but necessary for operation, and in the blocked state, a residual current flows, the voltage drop and the residual current should be as small as possible. The problem area "reduction of the voltage drop in the conductive state of the switching device" has already been treated in detail in the published German Patent Applications and Patents, Nos. 19 51 137; 21 27 956; 26 13 423; 27 11 877; 28 08 156; 29 22 309;and 33 20 975.
Otherwise, in addition to the 2-wire switching devices explained above, 3-wire switching devices are also known; therefore, electronic switching devices which have three switching device terminals. In these switching devices, the power supply voltage can be connected to two switching device terminals, while the load, to which, otherwise, the power supply voltage can be connected, can be connected to the third switching device terminal. In the 3-wire switching devices which are made as direct current devices, therefore, in 3-wire DC switching devices, it is distinguished between those which are plus-switching, and those which are minus-switching, depending on whether the third switching device terminal, and thus the load connected thereto, are switched through via the electronic switch to the switching device terminal which carries the plus potential of the power supply voltage, or the switching device terminal which carries the minus potential of the power supply voltage. There are also 4-wire DC switching devices which have a switching device terminal which is switched through by an electronic switch to the switching device terminal which carries the plus potential of the power supply voltage, and which have another switching device terminal which is switched through by another electronic switch to the switching device terminal which carries the minus potential of the power supply voltage. These 4-wire DC switching devices can, therefore, be used selectively as plus-switching or minus-switching, depending on to which of the two switching device terminals provided for this purpose the load is connected. However, there are also 3-wire DC switching devices which can be used selectively as plus-switching or minus-switching, and which can, therefore, be programmed accordingly.
But, 2-wire switching devices, as compared to 3-wire switching devices, of course, have not only the above described system-induced defects that, in the conductive state, a voltage drop occurs, and in the blocked state, a residual current flows, they also have the system-induced advantage that only two lines need be installed and connected. Nonetheless, 3-wire switching devices are used extensively, specifically when the system-induced defects of 2-wire switching devices, the voltage drop in the conductive state and residual current in the blocked state, cannot be accepted.
It was stated at the start that, in 2-wire switching devices, the sensor and evaluation circuit, on the one hand, and the electronic switch, on the other, are connected at least essentially in parallel. Conversely, it applies to 3-wire switching devices that the sensor and evaluation circuit, on the one hand, and the series connection of the electronic switch and the load on the, other hand, are connected in parallel. In both cases, for the sensor and evaluation circuit, a certain minimum voltage must be available, specially roughly 2.5 V. On the one hand, this minimum voltage in 2-wire switching devices determines the voltage drop which must occur in the conductive state of the switching device. On the other hand, this minimum voltage stipulates the minimum power supply voltage in the 3-wire switching devices.
In the meantime, it has been possible to implement 2-wire switching devices also in a polarized version, which in the conductive state have only a voltage drop of roughly 2.5 V (compare in this respect the contents of unpublished German Patent Application 199 05 170) Consequently, these 2-wire switching devices can now also be used where, to date, 2-wire switching devices due to the functionally-necessary voltage drop in the conductive state could not be used, rather 3-wire switching devices have been used. This applies especially to use of those switching devices in conjunction with system-programmable controls (SPS). The SPS standard calls for a residual voltage of 5 V as the maximum value, but in fact, a residual voltage of roughly 2.5 V is required.
If the voltage drop of the 2-wire switching devices in the conductive state is no longer an obstacle to the inherently desirable use of 2-wire switching devices, there still remains a barrier to the inherently desirable use of 2-wire switching devices where so far 3-wire switching devices have been used. Specifically, in the conductive state, the 2-wire switching devices require a load current of at least 5 mA, otherwise, as explained initially, in the blocked state, they require a residual current which is currently at least roughly 0.5 mA.